Make naming of release-branch configurable

Hi, and thanks for this great tool!

**Is your feature request related to a problem? Please describe.**
In our company our branchnaming is following gitflow-conventions. By this means, our (previously manually created) release-branches are prefixed with `release/`. We're also using github-actions to validate the branchnames created in our repositories using this pattern.

Now our problem is that the release-branches generated by release-please are conflicting with this, as they are prefixed with `release-` and aren't configurable. This naming is hardcoded in multiple places in `src/util/branch-name.ts`.

**Describe the solution you'd like**
What we would like to have is a config option (like there are for `--package-name` or `--token` for example), that is allowing users to change this naming. At first it would be enough to only make the "divider" configurable, but I think, if one is making something there configurable anyways, one could even make this whole prefix/base configurable. Some examples:

- `release-v1.2.3` (<-- as it is now)
- `release/v1.2.3` (<-- what we want)
- `rocket_v1.2.3` (<-- what some other person maybe wants 🤷)

**Additional context**
Because in `src/util/branch-name.ts` the naming "branchPrefix" is already in use, I guess using "branchBase" would be a possible naming (cli-flag then `--branch-base`).

I would also be willing to implement that, but want to make sure that this has a chance beeing merged afterwards...so I don't waste time 😅